This study aimed at investigating the impact of entering high-ability classes on the academic self-concept of high-ability primary girls in Singapore. Participants in this study are 91 Primary 4 girls, 30 high-ability pupils, and 61 pupils from classes that include high-, middle-, and low-ability pupils. This study utilized a mixed-method approach. The quantitative part of the study used the Academic Self-Concept Questionnaire (ASCQ) to measure the pupils' academic self-concept before and after they were streamed into high-ability classes. Findings indicate that the high-ability learners had a statistically lower academic self-concept score after streaming, whereas no significant difference could be found between the academic self- concept scores for the pupils from the mixed-ability classes before and after streaming had taken place. Narrative interviews revealed that high-ability students experienced greater pressure because of heightened competition in class. Implications of the study for educators are discussed.